In recent years, from the viewpoint of global environmental conservation, enhancement of the automotive fuel economy has been aimed on an automobile industry-wide basis in order to reduce the amount of CO2 output. Reduction of the weight of an automotive body by decreasing the thicknesses of constituent members (automotive part elements) is most effective in enhancement of the automotive fuel economy. Also, in order to ensure the safety of occupants in the event of a crash, it has been required to strengthen the automotive body and improve crash safety of the automobile body. From such viewpoints, high strength hot rolled steel sheets capable of ensuring the compatibility between the weight reduction and the safety have been used as automotive part elements and the usage thereof has increased year after year.
In the case where automotive parts are produced by using such high strength hot rolled steel sheets, some parts are produced by subjecting a coil-shaped high strength hot rolled steel sheet (hot rolled coil) to slitting in the longitudinal direction thereof to produce a hoop having a predetermined width and subjecting the resulting hoop to pressing to form into a predetermined shape. Here, if there are variations in the strength in the width direction of the hot rolled coil serving as the element for the part, various problems occur, for example, the strength required for part of hoops, among the slit hoops, is not reached, or cracking occurs in pressing of the hoop. Also, strength variations in the width direction of the hot rolled coil (sheet width direction) tend to increase as the strength of the steel sheet is enhanced.
Consequently, in particular, with respect to the high strength hot rolled steel sheet used in the state of being slit into the hoop, demands for steel sheets exhibiting excellent in-plane strength uniformity have been increased.
Various studies have been made on the technologies to enhance the strength of a steel sheet and enhance the in-plane strength uniformity previously.
As for the steel sheet production technology aiming at enhancing the strength, for example, Patent Literature 1 proposes a high strength hot rolled steel sheet, wherein the steel sheet composition is specified to be a composition containing, on a percent by mass basis, C: 0.07% or more and 0.13% or less, Si: 0.3% or less, Mn: 0.5% or more and 2.0% or less, P: 0.025% or less, S: 0.005% or less, N: 0.0060% or less, Al: 0.06% or less, Ti: 0.08% or more and 0.14% or less, and V: 0.15% or more and 0.30% or less in such a way that a formula (1) (Ti≥0.08+(N/14×48+S/32×48)) and a formula (2) (0.8≤(Ti/48+V/51)/(C/12)≤1.2) are satisfied and the steel sheet microstructure is specified to be a microstructure, which has a ferrite phase area ratio of 97% or more and in which Ti-V based fine carbides having an average particle diameter of less than 10 nm are dispersion-precipitated in the ferrite phase and the volume fraction of the fine carbides is 0.007 or more relative to the entire microstructure of the fine carbides.
Then, according to the technology proposed in Patent Literature 1, the high strength hot rolled steel sheet having a tensile strength of 980 MPa or more and exhibiting excellent workability is obtained by specifying the steel sheet microstructure to be substantially a ferrite single phase microstructure and dispersion-precipitating Ti-V based fine carbides having an average particle diameter of less than 10 nm in the ferrite phase. Also, according to the technology proposed in Patent Literature 1, the Ti-V based fine carbides can be precipitated stably by specifying the steel sheet microstructure to be a microstructure which satisfies the formula (1) (Ti≥0.08+(N/14×48+S/32×48)) and the formula (2) (0.8≤(Ti/48+V/51)/(C/12)≤1.2) described above and optimizing the coiling temperature in production of the hot rolled steel sheet and, thereby, the Ti-V based fine carbides are precipitated sufficiently even in an end portion in the hot rolled steel sheet width direction, where the amount of precipitation of carbides becomes insufficient easily, so that predetermined strength is obtained.
Likewise, as for the steel sheet production technology aiming at enhancing the strength, Patent Literature 2 proposes a high strength hot rolled steel sheet, wherein the steel sheet composition is specified to be a composition containing, on a percent by mass basis, c: 0.02% to 0.10%, Si≤2.0%, Mn: 0.5% to 2.0%, P≤0.08%, S≤0.006%, N≤0.005%, and Al: 0.01% to 0.1% and containing Ti in an amount satisfying Ti: 0.06% to 0.3% and 0.50<(Ti-3.43N-1.5S)/4C and the steel sheet microstructure is specified to be a microstructure which has an area ratio of low temperature transformed product and pearlite of 15% or less and in which TiC is dispersed in polygonal ferrite. Then, according to the technology proposed in Patent Literature 2, the high strength hot rolled steel sheet exhibiting excellent stretch flangeability and having tensile strength (TS) of 70 kgf/mm2 (686 MPa) or more is obtained by specifying the composition and the microstructure, as described above.
On the other hand, as for the technology to improve the stability of mechanical properties of a steel sheet, for example, Patent Literature 3 proposes a technology to produce a high strength steel sheet by subjecting a steel slab having a composition containing, on a percent by mass basis, C: 0.05% to 0.15%, Si: 0.06% to 0.7%, Mn: 1.0% to 2.5%, P: 0.01% to 0.05%, S: 0.0050% or less, Al: 0.01% to 0.10%, N: 0.005% or less, Nb: 0.01% to 0.10%, and Ta: 0.001% to 0.010% and further containing at least one of V: 0.10% or less and Ti: 0.100% or less to hot rolling, cold rolling, annealing, and temper rolling under predetermined conditions. Then, according to the technology proposed in Patent Literature 3, a stable precipitate can be precipitated by adding Nb and Ta at the same time and a high strength steel sheet exhibiting excellent stability of mechanical properties is obtained by using the steel slab, to which Nb and Ta have been added at the same time, and controlling the cooling condition after finish rolling in the hot rolling, the coiling temperature, and the annealing condition.
Meanwhile, Patent Literature 4 proposes a hot rolled steel sheet which contains c: 0.1% or less, Mo: 0.05% to 0.6%, and Ti: 0.02% to 0.10% and in which carbides containing Ti and Mo within the range satisfying Ti/Mo≥0.1 on an atomic ratio basis are finely dispersed into a ferrite microstructure substantially. Then, according to the technology proposed in Patent Literature 4, the steel sheet in which mechanical property variations are reduced, the strength in the coil width direction is uniform, and excellent mechanical property uniformity is exhibited is obtained by strengthening the ferrite microstructure with the fine precipitate having the above-described Ti/Mo atomic ratio.